In an electrohydraulic (EH) system, an electronically controlled valve is typically designed to have a certain current control characteristic. In other words, the current that is driven to the valve control solenoid will either directly or indirectly displace the valve spool. The spool may require a certain minimum amount of displacement before it enters its metering range, which is the range at which fluid starts to flow across the valve. This spool displacement, and the current required to get this displacement, are often designed to occur at a specific point. However, due to manufacturing tolerances, the imprecision of this “start of flow” or “cracking” point may not be adequate for a particular application.
The area of the opening or orifice allowing the flow of fluid through an electrohydraulic valve is controlled by a valve current. An electrohydraulic valve has a current dependent metering range between a cracking current and a saturation current. The cracking current is the valve current value at which the valve orifice is very slightly open allowing a very small flow of fluid through the valve. The saturation current is the valve current value at which the valve orifice is fully open allowing the maximum flow of fluid through the valve. The valve current control characteristic relating valve current to valve orifice size over the metering range is usually well characterized. A supplier valve current control characteristic is typically provided with a valve by the supplier. The supplier valve current control characteristic is usually accurate in shape but often needs to be offset to account for variances between valves, for example manufacturing variations, machining tolerances, etc., and impacts of valve environments.
A valve calibration routine can be implemented to estimate a start of flow point or cracking point to use as an offset for the supplier valve current control characteristic. One method involves a system with a load sensing hydraulic pump where the valve start of flow point can be determined by observing the pump output pressure while the hydraulic function downstream of the valve is in a stalled condition. In this scenario, the cracking of the valve induces a load sense signal that drives the pump up to pressure. However, this method cannot be used for calibration in an electrohydraulic system using a pump with an electronically controlled displacement. The cracking point and cracking current to produce this “start of flow” point is also very sensitive to manufacturing variations, machining tolerances, fluid temperatures, flow forces and various other factors.
It would be desirable to have a valve calibration system and method to calibrate an electrohydraulic valve that has greater stability and repeatability, and that can also be used for pumps with electronically controlled displacement.